The objective of this project is to build a parallel-processor based dosimetry system for use in radioimmunotherapy. Traditional methods for performing internal radionuclide dosimetry are deficient in that they cannot account for arbitrary sources and targets of radiation in the human body. The Monte Carlo method, whereby one simulates the physical decay of radioactivity and builds up a statistical answer with respect to dose distribution, is one alternative that can be adapted to estimate tumor dose. Monte carlo solutions, however, are extremely time consuming- a factor which may prove prohibitive in radioimmunotherapy. Restrictions on calculation time are necessary because a human anti-mouse antibody response can develop within 7-14 days of an initial diagnostic infusion, effectively changing a patient's antibody kinetics and negating the utility of a diagnostic study for therapeutic planning. Parallel processors provide an inexpensive solution to achieving the computation power necessary to arrive at dosimetry estimates in a cost effective manner. The specific aims of this proposal are: (1) to create a desktop dosimetry system for radioimmunotherapy that runs under a graphical user interface on a common microcomputer; (2) to determine optimal algorithms for the calculation of dosimetry estimates; and (3) to generalize the radiation source geometry to include external photon sources. Software will be written in C++ to perform data input, mathematical modeling, time-activity curve integration, and dose estimation using a graphical user interface. Radiation transport code will be written in the Occam language to execute on a resident parallel processor array. Timing benchmarks will be measured and compared to previously published execution times. Two algorithms that promise a significant decrease in computation time will be investigated for efficiency and accuracy. A radiation source geometry class will be created to extend the utility of the dosimetry system to problems involving external sources of radiation. The system will provide greater precision of tumor dose estimates, as well as functioning as a general dosimetry tool for use in radiation dosimetry.